The present invention relates to methods of operating, in particular for shutting down and starting of a fuel cell system in such a way, that degradation of performance over time is minimized. The invention further concerns a corresponding fuel cell system.
Electrochemical fuel cells of the above-mentioned type convert reactants, typically a stream of hydrogen and a stream of oxygen, into electric power and water. Proton exchange membrane fuel cells (PEMFC) generally comprise a solid polymer electrolyte membrane disposed between two porous electrically conductive electrode layers so as to form a membrane electrode assembly (MEA). In order to induce the desired electrochemical reaction, the anode electrode and the cathode electrode each comprise one or more catalyst. These catalysts are typically disposed at the membrane/electrode layer interface.
At the anode, the hydrogen moves through the porous electrode layer and is oxidized by the catalyst to produce protons and electrons. The protons migrate through the solid polymer electrolyte towards the cathode. The oxygen, for its part, moves through the porous cathode and reacts with the protons coming through the membrane at the cathode. The electrons travel from the anode to the cathode through an external circuit, producing an electrical current.
Typical proton exchange membrane fuel cell stacks include a pair of end plate assemblies and a plurality of fuel cell assemblies. Reactant and coolant fluid streams are supplied to, and exhausted from, internal manifolds and passages in the stack via inlet and outlet ports in the end plates.
Each fuel cell assembly includes an anode flow field plate, a cathode flow field plate and a Membrane Electrode Assembly (MEA) interposed between these flow field plates. Anode and cathode flow field plates are made out of an electrically conductive material and act as current collectors. As the anode flow field plate of one cell sits back to back with the cathode flow field plate of the neighboring cell, electric current can flow from one cell to the other and thus through the entire stack. Other prior art fuel cell stacks are known in which individual cells are separated by means of cooling elements between flow field plates or by means of a single bipolar flow field plate instead of by separate anode and cathode flow field plates.
Flow field plates further provide a fluid barrier between adjacent fuel cell assemblies so as to keep reactant fluid supplied to the anode of one cell from contaminating reactant fluid supplied to the cathode of another cell. At the interface between MEA and plates, fluid flow fields direct the reactant fluids to the electrodes. A fluid flow field typically comprises a plurality of fluid flow channels formed in the major surfaces of flow field plates facing the MEA. One purpose of a fluid flow field is to distribute the reactant fluid to the entire surface of the respective electrodes, namely the hydrogen on the anode side and the oxygen on the cathode side.
One known problem with PEMFCs is the progressive degradation of performance over time. Actually, long-term operation of solid polymer fuel cells has been proven, but only under relatively ideal conditions. In contrast, when the fuel cell has to operate in a wide range of conditions, as it is the case for automotive applications in particular, the ever-changing conditions (often modeled as load cycling and start-stop cycles), have been shown to reduce durability and lifespan drastically.
Different types of non-ideal conditions have been identified in the literature. A first of these conditions is referred to as “high cell voltage”; it is known that exposing a fuel cell to low or zero current conditions, leads to higher degradation rates in comparison to operation at an average constant current. A second non-ideal condition is “low cell voltage”; it is further known that drawing a peak current from the fuel cell also leads to increased degradation rates. It follows from the above that, in order to preserve the lifespan of a fuel cell, it is preferable to avoid both “high cell voltage” and “low cell voltage” operating conditions. In other words, the fuel cell should be operated only in a limited voltage range.
In order to cope with the abrupt changes in load that are typical for automotive applications, an electrochemical energy storage unit, such as a battery or a super capacitor, is usually associated with the fuel cell. The battery can work as a buffer: supplying electric power when there is a peak in the load and, conversely, storing excess electric power in case of low or zero load conditions. In principle, such an arrangement allows operating the fuel cell in the desired limited voltage range. However, once the battery is completely charged, it obviously ceases to be available for storing the excess electric power supplied by the fuel cell. A known solution to this last problem is simply to shut down the fuel cell until the level of charge of the battery reaches a lower threshold. However, start-stop cycles also contribute to the degradation of performance over time.
Document U.S. 2008/0038595 A1 for instance discloses a method for shutting down an electricity supply system comprising a fuel cell. This known shutting down procedure is activated on reception of a stop signal and comprises an initial stage, during which the supply of oxygen is interrupted, a consumption stage, during which a sustained current is drawn from the fuel cell, a neutralization phase, during which the oxygen feed circuit is opened to the atmosphere, and a final state, during which the supply of hydrogen is interrupted. Further care is taken, that the hydrogen pressure is never lower than atmospheric pressure, which is the pressure of the oxygen circuit at the end of the shut-down sequence.